In the early 1990s, [Me2Si(Me4C5)NtBu]TiCl2 (Constrained-Geometry Catalyst, hereinafter abbreviated as CGC) was reported by Dow Co. (U.S. Pat. No. 5,064,802), and superior aspects of the CGC in a copolymerization reaction of ethylene and alpha-olefin may be summarized by the following two points when compared to commonly known metallocene catalysts. (1) At a high polymerization temperature, high activity is shown and a polymer having a high molecular weight is produced, and (2) the copolymerization ability of alpha-olefin having large steric hindrance such as 1-hexene and 1-octene is excellent. In addition, a variety of characteristics of CGC upon polymerization are gradually becoming known, and thus thorough research into synthesis of derivatives thereof to serve as a polymerization catalyst is ongoing in academic and industrial fields.
A Group 4 transition metal compound having one or two cyclopentadienyl groups as a ligand may be used as a catalyst for olefin polymerization by activating it with methylaluminoxane or a boron compound. Such catalyst shows unique characteristics that a traditional Zeigler-Natta catalyst cannot realize.
That is, a polymer obtained by using such catalyst has a narrow molecular weight distribution and higher reactivity for a second monomer such as alpha-olefin or cycloolefin, and distribution of the second monomer in the polymer is even. Furthermore, it is possible to control the stereoselectivity of the polymer in the polymerization of alpha-olefin by changing the substituent of the cyclopentadienyl ligand in the metallocene catalyst, and it is easy to control the degree of copolymerization, the molecular weight, and the distribution of the second monomer upon copolymerization of ethylene and other olefins.
Meanwhile, since the metallocene catalyst is more expensive than the Zeigler-Natta catalyst, it must have good activity for its economic cost. If the metallocene catalyst has high reactivity for the second monomer, there is an advantage that a polymer including a large amount of the second monomer may be obtained by using only a small amount of the second monomer.
Many researchers have studied various catalysts, and as a result, it is proven that a bridged catalyst generally has high reactivity for the second monomer. The bridged catalysts developed until now may be classified into three types according the type of the bridge. One is a catalyst of which two cyclopentadienyl ligands are connected with an alkylene dibridge by the reaction of an electrophile such as an alkyl halide and indene or fluorene, another is a silicone-bridged catalyst of which the ligands are connected with —SiR2—, and the other is a methylene-bridged catalyst which is obtained by the reaction of fulvene and indene or fluorene.
Among the above attempts, however, very few catalysts have been practically applied in commercial factories, and thus preparation of catalysts showing more improved polymerization performance is still required.